The present invention relates to power factor control circuits for alternating current reactive loads and, more particularly, to a low cost power factor control circuit for AC induction motors.
Power factor control circuits are well known in the art and are used to improve efficiency of AC motor drives. In particular, AC induction motors generally operate at a speed which is related to the frequency of the applied excitation and independent, within limits, of the applied voltage and load. Accordingly, under light load conditions, the motor can run at constant speed but draw more current than is actually required to produce power to drive the light load. The motor is, therefore, inefficient at light load and power factor deteriorates. The practical solution to improve efficiency, i.e., power factor, is to adjust the voltage applied to the motor so that the applied voltage is a function of loading. Most AC motor power factor control circuits achieve this function by modulation of the voltage applied to the motor, i.e., by removing voltage from the motor for at least some portion of each half-cycle of the AC voltage waveform.
In general, circuits used with AC motors for power factor control employ some form of controllable electronic switching device, such as a triac, connected in series circuit between an AC power source and each phase winding of the motor. Monitoring circuit units (MCU) are then used to determine the zero crossings of the motor current and motor voltage, the difference between the zero crossings of voltage and current representing the phase shift, which is proportional to power factor. A microcontroller uses the phase shift measurement to adjust or control the triac conduction times so as to vary the duty cycle of the voltage applied to the motor in a manner to reduce the phase shift and thus improve motor efficiency.
A disadvantage of the prior art circuits for power factor control is the necessity of identifying the zero crossings of voltage and current. In many instances the current in the AC motor is characterized by noise and other oscillations which may create multiple zero crossings each time current reverses. Such current variations are reflected onto the voltage waveform and can provide similar difficulty in identifying a true zero crossing. As a consequence, circuits for determining current and voltage zero crossings may be more complex than desired and increase the cost of implementing power factor controls.